Flat or low-slope roofs are often covered with multi-layered roofing systems. These roofing systems often include a roof deck, an insulation layer, and a protective, weather-resistant membrane. In some situations, a coverboard is also employed. In many situations, insulation boards are typically adhered by mechanical fasteners or bonding adhesives directly to a roof deck, which is most commonly constructed of concrete or steel. These insulation boards are typically closed-cell foams that include polyurethane or polyisocyanurate cellular materials with an insulating gas trapped within the cells. The insulation boards are then covered with the weather resistant membrane.
The foam insulation boards are typically low density cellular structures. The low density stems from two primary considerations. The first is cost because lower density cellular structures employ less material. Also, and often more important, the insulation value of the board can be improved with lower density structures. While a density of zero would be ideal (i.e., a vacuum), a certain degree of cellular structure—which gives rise to the density—is required to maintain the integrity of the insulation boards. Particularly, a foam core density of greater than about 1.5 pounds per cubic foot (pcf) is needed to maintain strength, and a core density of less than about 2 pcf is conventional for cost and insulation considerations.